1. Field of the Invention
This invention relates to a liquid sensor, and more particularly to a liquid sensor for non-contact discrimination between the presence and the absence of an electrolyte solution flowing through a pipeline or tube.
2. Description of the Prior Art
There have been known liquid sensors for checking whether water containing an electrolyte dissolved therein is flowing through a pipeline or not.
FIG. 6 shows an example of a conductivity sensor for detecting the presence or absence of an electrolyte solution flowing through a pipeline, according to the prior art. In the figure, the pipeline 2 through which the electrolyte solution 1 flows intermittently is provided with a pair of electrodes 3a and 3b disposed at a predetermined interval to penetrate into the pipeline 2, and a predetermined voltage is impressed on the electrodes from a power supply, not shown, through respective lead wires 4a and 4b.
As is well known, an aqueous solution containing an electrolyte shows an extremely high electric conductivity, whereas air has an extremely low conductivity.
When the electrolyte solution flows through the pipeline 1, therefore, the electric resistance between the electrodes 3a and 3b is lowered, and by detection of the lowering it is possible to find the presence of the electrolytic solution in the pipeline 1.
When the electrolyte solution is not flowing through the pipeline 1, on the other hand, the conductivity between the electrodes 3a and 3b is low because of the presence of air, and the electric resistance between the electrodes is extremely high, as compared with that in the presence of the electrolyte solution flowing through the pipeline 1.
Accordingly, it is possible to check whether the electrolyte solution is present or absent in the pipeline 2, based on variations in the electric resistance between the electrodes 3a and 3b.
As a contrast to the above conductivity sensors of the type in which electrodes make direct contact with an electrolyte solution, there have been known non-contact type optical liquid sensors, one example of which is illustrated in FIG. 7.
The non-contact type sensor, also intended for discrimination between the presence and the absence of the liquid flowing through the pipeline 2, comprises a light emitting diode (LED) 5 and a phototransistor 6 disposed opposite to each other on the wall surface of the pipeline 2. Light is transferred between the LED 5 and the phototransistor 6, and the presence of the electrolyte flowing through the pipeline 2 is distinguished based on the difference between the photoelectric output in the presence of the solution and the photoelectric output in the absence of the solution.
Namely, in the absence of the solution in the pipeline 2, the light emitted from the LED 5 is received, substantially as it is, by the phototransistor 6, which generates an extremely high photoelectric output. In the presence of the electrolyte solution flowing through the pipeline 2, on the other hand, the light from the LED 5 is attenuated by the solution itself and by the turbidity, color or the like, so that the photoelectric output of the phototransistor 6 is extremely low, as compared with the output in the absence of the solution in the pipeline. Thus, it is possible to detect whether the solution is flowing through the pipeline 2 or not.
However, the contact-type liquid sensor as shown in FIG. 6 involves the direct contact of the electrolyte solution with the electrodes 3 and, therefore, there is the possibility that various chemical substances contained in the solution 1 may deposite on the electrodes 3 or the electrodes 3 themselves may be dissolved, resulting in marked deterioration of the performance of the sensor.
In view of the above, plating the electrodes 3 with copper, silver or other precious metal may be contemplated. The plating, however, leads to a large increase in the cost of the sensor.
There may arise a further problem, particularly where the electrolyte solution 1 intermittently flows and stops flowing, repeatedly, at relatively short intervals of time. Namely, a detection result similar to that obtained in the presence of the solution 1 flowing through the pipeline may be produced in the absence of the solution, due to the drops of the solution remaining in the pipeline 2. For obviating such a situation, it is necessary to provide a wide spacing between the electrodes, which causes a lowering in the sensitivity of detection and a limitantion on the control range.
On the other hand, the optical type sensor illustrated in FIG. 7, which is capable of non-contact detection, has the problem that the pipeline 2 must be formed of a transparent material for permitting the transmission of light through the walls of the pipeline. Besides, the attenuation of the light transmitted through the solution constitutes the condition for discrimination between the presence and the absence of the solution. Therefore, where the solution is a colorless transparent solution, the attenuation coefficient is extremely low and, accordingly, there is a possibility of such a malfunction that the solution actually flowing through the pipeline may be judged absent.